Process for manufacturing shell membrane force and deflection sensor

ABSTRACT

A sensor for force is formed from an elastomeric cylinder having a region with apertures. The apertures have passageways formed between them, and an optical fiber is introduced into these passageways, where the optical fiber has a grating for measurement of tension positioned in the passageways between apertures. Optionally, a temperature measurement sensor is placed in or around the elastomer for temperature correction, and if required, a copper film may be deposited in the elastomer for reduced sensitivity to spot temperature variations in the elastomer near the sensors.

The present patent application is a continuation of Ser. No. 12/100,417filed Apr. 10, 2008, now issued as U.S. Pat. No. 7,903,907.

The present invention was developed under National Aeronautics and SpaceAdministration (NASA) Contracts #NNJ05JC02C and NNJ06JA36C. Thegovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a force and deflection sensor. Inparticular, the invention relates to an flexible shell formed with anelastomer having passageways formed by apertures in the shell, with anoptical fiber having one or more Bragg gratings positioned in thepassageways for the measurement of force and deflection.

2. Description of the Related Art

Future robots are expected to free human operators from difficult anddangerous tasks requiring high dexterity in various environments. Oneexample is an extra-vehicular repair of a manned spacecraft that wouldotherwise require hazardous work by human astronauts. Another example isrobotic surgery in which accurate manipulation is crucial. Operatingcomplicated tools and performing delicate tasks require a manipulator ofgreat precision and coordination. Therefore, force sensing is one of themost critical requirements for this type of robot control. Typically,robots have a modest number of mechanical sensors, often associated withactuators or concentrated in a special device such as a force sensingwrist. As a result, robots often poorly identify and respond tounexpected and arbitrarily-located impacts.

One object of the invention is a light-weight, rugged appendages for arobot that features embedded sensors so that the robot can be more awareof both anticipated and unanticipated loads in real time. A particularclass of optical sensors, Fiber Bragg Grating (FBG) sensors, ispromising for space robotics and other applications where highsensitivity, multiplexing capability, immunity to electromagnetic noise,small size and resistance to harsh environments are particularlydesirable. In addition, the biosafe and inert nature of optical fibersmaking them attractive for medical robotics. FBGs reflect light with apeak wavelength that shifts in proportion to the strain to which theyare subjected. This wavelength shift provides the basis for strainsensing with typical values for the sensitivity to an axial strain beingapproximately 1.2 pm/microstrain at 1550 nm center wavelength. Incombination with a prior art FBG interrogator, submicrostrain resolutionmeasurements are possible. In addition, the strain response is linearwith no indication of hysteresis at temperatures as high as 370° C. and,with appropriate processing, to over 650° C. Multiple FBG sensors can beplaced along a single fiber and optically multiplexed. FBG sensors havepreviously been surface attached to or embedded in metal parts andcomposites to monitor stresses.

SUMMARY OF THE INVENTION

A sensor is formed from a thin shell of flexible material such aselastomer to form an attachment region, a sensing region, and a tipregion. In one embodiment, the sensing region is a substantiallycylindrical flexible shell and has a plurality of apertures formingpassageways between the apertures. Optical fiber is routed through thepassageways, with sensors located in the passageways prior to theapplication of the elastomeric material forming the flexible shell.Deflection of the sensor, such as by a force applied to the contactregion, causes an incremental strain in one or more passageways wherethe optical fiber is located. The incremental strain results in a changeof optical wavelength of reflection or transmittance at the sensor,thereby allowing the measurement of force or displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an elastomeric sensor.

FIG. 2A is a cross section view of a sensor.

FIG. 2B is a section view of the sensor of FIG. 2A

FIG. 3A is a front view of a sensor with a force applied to the tip.

FIG. 3B is a side view of a sensor with a force applied to the tip.

FIG. 3C is a front view of a sensor with a force applied to a sensingregion.

FIG. 3D is a side view of a sensor with a force applied to a sensingregion.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 4J, 4K show process steps forforming an elastomeric sensor.

FIGS. 5A and 5B are plots of wavelength shift vs. applied force.

FIGS. 6A and 6B are plots of dynamic impulse response and its fastFourier transform.

FIG. 7 is a plot of force measurement accuracy for temperaturecompensated and temperature uncompensated configurations.

FIG. 8A is a cross section view of deformation at the sensor region.

FIGS. 8B and 8C are superposition views of the force-induced deformationof FIG. 8A.

FIG. 9 is a plot of a strain ratio versus applied force along variouspoints from the sensor joint.

FIG. 10 is a top view of a sensor at a region of applied force.

FIG. 11 is a plot of sensor output for the sensors of FIG. 10.

FIG. 12 is a block diagram for a wavelength discriminator coupled to aseries connection of FBGs.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of an example embodiment of a sensorfinger 100 having an attachment region 102, a hollow sensing region 104,and a hollow tip region 106. The sensor is not limited in itsdimensions, but in one example embodiment, the attachment region extentis approximately 35 mm, the sensing region extent is approximately 65mm, and the tip region extent is approximately 20 mm with a cylindricaldiameter of 35 mm. FIG. 2A shows a simplified cross section view of thesensor of FIG. 1, which includes a polyurethane shell 210 forming thetip region and sensing region, transitioning to a rigid attachmentregion including mounting holes 230. The shell 210 has a fiber opticcable embedded in it, which may route through passageways of the sensingregion and tip region in any manner through the shell 210, and having aplurality of strain sensors formed as Bragg gratings in particularregions such as 214 and 218 of the shell 210. FIG. 2B shows a sectionA-A of FIG. 2A, including four such strain sensors S1 214, S3 218, S2220, and S4 222. A control sensor S5 215 is positioned at the center,and is not exposed to strain, but measures temperature for use incompensating temperature effects from strain sensors S1, S2, S3, S4.

The exoskeletal structure of the shell 210 is light weight whilemaintaining relatively high strength. Since the sensing region structuredeforms not only locally but globally depending on the location of forceapplication, the sensor finger 100 is able to measure and localizeapplied forces. This is useful for both grasp force measurement andcollision detection.

In one embodiment of the invention, the sensing region 104 has ahexagonally patterned shell. This pattern allows the structure toconcentrate stresses and strains on the narrow ribs, facilitatesembedded sensor placement and has an added effect of amplifying thesensor signal. Although two other regular polygons, triangles andsquares, can also be used exclusively to form the shell pattern, thehexagon minimizes the ratio of perimeter to area. In addition, thehexagonal cells avoid sharp interior corners which could reduce thefatigue life. In summary, the hexagonal structure can minimize theamount of material for fabrication and the weight of the part whileproviding high structural strength. Although shown as a regular array ofhexagonal aperture patterns, the sensor passageways could be formed manydifferent ways and with various combinations of apertures, includingpairs of apertures with a sensor placed therebetween, an array ofsensors with circular symmetry, radial symmetry, or circumferentialsymmetry, and the passageways containing the sensors may have anyorientation with respect to the axis of the sensing region 104.

Polymer structures unavoidably experience greater creep than metalstructures. Creep adversely affects the linearity and repeatability ofthe embedded sensor output, both of which are mainly dependent on thestiffness and resilience of the structure. In addition, thermal changescan affect the FBG strain sensor outputs. A copper mesh 212 can beembedded into the outside of the shell, to reduce creep and providethermal shielding. The high conductivity of copper expeditesdistribution of heat applied from outside the shell and creates a moreuniform temperature gradient inside the shell.

Additional sensors provide more information and make the system morereliable. In an example embodiment, the force information obtained fromthe system includes longitudinal location, latitudinal location,magnitude of applied force, and orientation of the force vector. Forsimplicity, it will be assumed that forces are applied only in a normaldirection to the surface. Since this assumption reduces the number ofunknowns to three, a minimum of three linearly independent sensors areneeded. In the present example, four strain sensors are embedded in theshell. Optimal sensor locations may be determined through the use offinite element analysis of the sensor shell. FIG. 3 shows straindistributions when different types of forces are applied to the shelland to the fingertip. Strain is most concentrated at the top of theshell where it is connected to the joint. Therefore, four sensors wereembedded at 90° intervals into the first rib of the shell, closest tothe joint, as shown in FIG. 2B, which also shows the center referencetemperature sensor S5 216, which may be partially enclosed in ashielding cylinder 232 such as stiff copper or other structure tominimize short-term effects such as air currents.

Since embedded FBG sensors are sensitive to temperature change as wellas strain change, it is necessary to isolate thermal effects frommechanical strains. Among the temperature compensation methods availableare the dual-wavelength superimposed FBG sensors, saturated chirped FBGsensors, and an FBG sensor rosette. In contrast, a simpler method shownin the present example embodiment is the use of an isolated, strain-freeFBG sensor S5 216 to directly measure the thermal effects. Subtractingthe wavelength shift of this temperature-compensation sensor from thatof any other sensor corrects for the thermal effects on the latter. Animportant assumption in this method is that all sensors are at the sametemperature. The example embodiment of FIGS. 2A and 2B shows onetemperature compensation sensor S5 216 in the hollow area in the middleof the shell 210 as shown in FIG. 2. Although the temperaturecompensation sensor S5 216 is physically removed from the strainsensors, the copper heat shield 232 is expected to create a more uniformtemperature gradient inside the shell.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 4J, and 4K shows the sequenceof steps of a modified SDM process for prototype fabrication, which maybe examined in combination with the below process steps:

Shell (Sensor Region) Part Fabrication:

FIG. 4A shows the preparation of a silicone rubber inner mold 402 withoptical fiber 404 with FBG sensor 406 placed with the FBG sensors 406 indesired measurement passageways formed in the mold. Typically, multipleFBGs operating at the same or different wavelengths are employed on asingle fiber, although only one is shown for clarity.

FIG. 4B shows the inner mold 402 with optical fiber 404 wrapped withcopper screen mesh 410. In one embodiment of the invention, the screenmesh has a matching hole pattern for the apertures formed as pedestalsinto inner mold 402, and the pedestals may also have a formed lip tosupport the mesh 410 at a desired height above the cylindrical innerdiameter of the inner form 402.

FIG. 4C shows a form void 414 produced by the gap between the inner mold402 which has the screen 410 applied, and the outer form 416. Liquidpolyurethane is then poured into the form gap 414.

FIG. 4D shows the formed sensing region 418 after curing of thepolyurethane and removal of the inner and outer molds with embeddedoptical fiber 404 and copper mesh (not shown).

[Step 2] Tip Region Part Fabrication

FIG. 4E shows the preparation of the silicone rubber inner mold 430 andwax outer mold 432 with the copper mesh placed in the form void 431.

FIG. 4F shows the liquid polyurethane poured into the form void 431produced by the inner 430 and outer 432 molds.

FIG. 4G shows the placement of the cured sensing region 418 from Step 1into the uncured polyurethane void 434 of the tip region mold, therebyjoining the formed sensing region 418 with the tip region.

FIG. 4H shows the tip and sensor regions after removal from the moldwith the polyurethane cured, including strain sensor fiber 404.

[Step 3] Attachment Region Part Fabrication

FIG. 4I shows the preparation of the outer mold with the placement ofthe temperature compensation sensor structure including measurementfiber 444 with grating 446 located in shield cylinder 448, all of whichis placed in base mold 442 with liquid polyurethane 440.

FIG. 4J shows the cured shell and fingertip parts placed into theuncured polyurethane.

FIG. 4K shows the completed sensor after removal of the outer mold whenthe polyurethane cures, including temperature measurement fiber 444 andstrain measurement fiber 404.

The series of FIGS. 4A through 4F show just one embodiment for the stepsof the Shape Deposition Manufacturing (SDM) process for the fingerprototype fabrication. As is known in the prior art, it is difficult tomake hollow three-dimensional parts using conventional SDM processes,since only the top of the part is accessible for machining. The outermold 431 and 416 may be formed from hard wax to maintain the overallshape. In contrast, the inner mold 402 can be hollow and made of softsilicone rubber, which can be manually deformed and removed when thepolyurethane is cured. The strain sensors and copper mesh can beembedded during these steps. The second series of steps is related tofingertip casting, which uses a separate mold and occurs after the shell418 is fully cured. As it cures, the polyurethane for the fingertip 434bonds to the cured polyurethane of the shell 418. In the final step, theattachment joint including temperature compensation structure 446 and448 are cast. As with the fingertip, the joint bonds to the cured shell.Since the joint is not hollow, an inner mold is not needed during thisstep. Since the joint has no copper mesh, it was cast using a hardpolyurethane (such as Task 9, Smooth-On, Easton, Pa., USA) to reducecreep, while the shell and fingertip can be cast from softerpolyurethane (such as Task 3, Smooth-On, Easton, Pa., USA).

To evaluate the resulting structure, three different sets of tests werecarried out to evaluate the static, dynamic, and thermal performance ofthe prototype. The static tests show how linear and repeatable thesystem is, the dynamic tests show how responsive the system is, and thethermal tests show how well the system compensates for errors caused bytemperature change.

Static Tests Static forces were applied to two different locations onthe finger: lattice shell region 104 and tip region 106. FIGS. 5A and 5Bshow the responses of two of the four sensors for a normal force appliedin the sensing region and tip region, respectively. Applying a force tothe shell yielded sensitivities of 0.024 nm/N and −0.0044 nm/N forsensor 51 and S3, respectively. The optical system can resolvewavelength changes of 0.5 pm or less, corresponding to 0.015 N or lessfor the minimum detectable force change. Note that the 51 sensor, beingon the same side of the shell as the contact force, has a much highersensitivity to it. Applying a force to the tip yielded sensitivities of0.032 nm/N and −0.029 nm/N. In this case, the location of the forceresults in roughly equal strains at both sensors. For a given location,the ratio of the two sensor outputs is independent of the magnitude ofthe applied force. The plots of FIGS. 5A and 5B shows a maximum of 5.3%and 3.9% deviations from linear responses for shell and tip tests,respectively.

Characterization of the dynamic response of the sensorized fingers canbe seen in FIGS. 6A and 6B, which show the impulse response (expressedas a change in the wavelength of light reflected by an FBG cell) and itsfast Fourier transform (FFT). The impulse was effected by tapping on thefinger with a light and stiff object. The FFT shows a dominant frequencyaround 167 Hz which is a result of the dominant vibration mode of theelastomer structure.

FIG. 7 shows a typical thermal test results. Over a three minute period,the fingertip was loaded and unloaded while the temperature wasdecreased from 28.3 C to 25.7 C. The ideal (temperature invariant)sensor output is indicated by the dashed line 806. Experiment resultsshow that use of the temperature compensation sensor of plot 804 reducesthermal effects, compared to the uncompensated plot 802.

Longitudinal localization requires some understanding of structuraldeformation of the shell. FIG. 8A shows simplified two-dimensionaldiagrams of one embodiment of the invention. When a force is exerted ata certain location, as shown in FIG. 8A, the structure will deform andsensors A and B will measure strains ε_(A) and ε_(B), respectively asindicated. This situation can be decomposed into two separate effects,as shown in FIGS. 8B and 8C. By superposition, ε_(A)=ε₁+ε₂ and ε_(B)=ε₃.Therefore, if the ratio of ε_(A) to ε_(A) is known, it is possible toestimate D, the longitudinal location of the force. FIG. 9 shows theplot of experimental ratios of ε_(A) to ε_(B) as a function of D. Thereis some ambiguity in the localization, since two values of D result inthe same ratio. However, if we let d0 be the distance at whichε_(A)/ε_(B) is minimized, and we restrict operation to the region d>d0,the ambiguity can be resolved. Further, if the sensor locations arepositioned closer to the other surface of the shell, d0 approaches 0 andit is possible to localize an applied force closer to the joint.

Latitudinal location can be approximated using centroid and peakdetection, and only one point contact force is assumed in this method.FIG. 10 shows a cross sectional view of the finger with four strainsensors and an applied contact force indicated, and FIG. 11 shows itscorresponding sensor signal outputs. The two sensors closest to theforce location will experience positive strains (positive sensoroutput), and the other two sensors negative strains (negative sensoroutput), regardless of the longitudinal location of the force, if d>d0.However, since all the sensor signals must be non-negative to use thecentroid method, all signal values must have the minimum signal valuesubtracted from them. Then, it is possible to find the angularorientation ⊖ of the contact force:

$\theta = {\frac{{\Sigma\phi}_{i}S_{i}^{\prime}}{\Sigma\; S_{i}^{\prime}} - \alpha}$

for i=1, 2, 3, 4, where S′=Si−min{S1,S2,S3,S4}, φ1=α, andφ_(k)=φ_(k-1)+π/2, for k=2, 3, 4 (if φ_(k)≧2π, φ_(k)=φ_(k)−2π), Si isthe output signal from sensor i, and α is the clockwise angle betweensensor 1 and the sensor with the minimum output signal value. Thismethod produced errors less than 2°, corresponding to less than 0.5 mmon the perimeter, and an offset of 1.5° in the FEM simulation.

The FBG sensors can be interrogated using a system such as IFOS I*Sense,described in U.S. Pat. Nos. 7,127,132, 6,895,132, 6,788,835, 6,751,367,and 6,597,822, which are incorporated herein by reference. This type ofinterrogator relies on parallel photonic processing whereby multiplesensors are placed in series on a single fiber, which in combinationwith the ability to place sensors over the many channels of the sensor100, has the near-term potential to combine high channel counts (>100sensors on a single fiber), high resolution (sub-microstrain), and highspeed (>5 kHz) with a miniaturized footprint. As previously discussed,the application of strain on each FBG produces a shift in the wavelengththat is linearly proportional to the strain. An FBG interrogator is usedto precisely measure, for each FBG, the reflected wavelength shift andthus the strain applied to that FBG. Interrogators can be tunable(examining each FBG sequentially) or parallel processing in nature—thelatter approach, which forms the basis of the preferred interrogatorsystem, has advantages in terms of speed particularly when dealing withmany sensors.

One example of a fiber interrogation system is shown in FIG. 12. Abroadband source 1250 sends light through the optical circulator orsplitter 1252 to an array of series FBGs 1202, each of which 1204, 1206,1208, 1210 reflects a different Bragg wavelength. The reflected light1214 is then returned through the optical circulator 1252 to thephotonic processor which both demultiplexes the light using AWG filter1252 coupled to detectors 1244 through 1250 and provides the basis for aratiometric approach to measuring each of the returned wavelengthsthrough conversion to different signals in various outputs from themulti-channel photodetector array. A controller 1242 containingelectronics and software compares the ratios of optical stimulationacross the detectors to provide the final conversion of the arrayedsignals to wavelength and eventually the strain to which each FBG issubjected. One type of device for generating a plurality of outputs forratiometric measurement is a specially formed arrayed waveguide grating(AWG) 1252, which may have filter response skirts which are optimizedfor wavelength discrimination, rather than the typical purpose ofisolating adjacent wavelengths.

While the above description describes examples for particularembodiments of the invention, there are many different ways in which theinvention can be practiced. Although the elastomeric sensor used a rapidprototyping process utilizing polyurethane, other variations of shapedeposition manufacturing can be used to support the fabrication ofhollow, plastic mesh structures with embedded components. In the presentembodiments, the fiber optic sensors were embedded near the base of acylindrical shell with hexagonal elements for high sensitivity toimposed loads, although the sensors could be placed in other locationsin the sensing region. With more precise location of the sensors, orcalibration of a particular sensor to a particular interrogator, highersensitivities and accuracies are possible. As the frequency limit isimposed by the mechanical finger system, other materials can be used incapturing the sensors to allow the measurement of dynamic strains tofrequencies of 5 kHz or more.

The 80 mesh 0.0055″ dia copper wire mesh embedded in the structurereduces the amount of viscoelastic creep and provides thermal shielding.A single FBG temperature compensation sensor at the center of the hollowfinger helps to reduce the overall sensitivity to thermal variations.However, the central sensor is sufficiently distant from the exteriorsensors that changes in temperature can produce noticeable transientsignals. This effect can be reduced using a larger number of sensors andlocating thermal compensation sensors near the exterior of thestructure, where they undergo the same transient thermal strains as theother sensors.

For simplicity, a 4 strain sensor with orthogonal axis was described.With a larger number of sensors, more accurate contact localization ispossible. Increasing the total number of sensors is relativelystraightforward as multiple FBG sensors can be located along the samefiber with optical multiplexing.

Furthermore, while the FBG strain and temperature sensors are describedas single (longitudinal) axis sensors in single-core glass fiber, it ispossible to use bend sensors based on multi-core fiber supporting FBGs,as well as the use of polymer optical fiber Bragg grating sensors inflexible robotic skins, and eventually a multiplicity of multiplexedphysical and chemical fiber-optic sensors.

1. A method for manufacturing a sensor having: a first step of placingan optical fiber into a cylindrical inner shell form, said inner shellform having an array of posts directed substantially outward, saidoptical fiber routed in the gaps between said posts; a second step ofplacing the optical fiber and cylindrical shell form into an enclosingouter shell form which is in contact with at least one of said outwarddirected posts, where a substantially cylindrical void is formed betweensaid inner shell form and said outer shell form; a third step of fillingsaid void with an elastomeric material to thereby create a sensor middlepart; a fourth step of forming a cap and an attachment; a fifth step ofattaching said cap on one end of said sensor middle part and attachingsaid attachment on the opposite end of said sensor middle part.
 2. Themethod of claim 1 where said fourth step cap and said attachment areformed from the same elastomeric material as said third step sensormiddle part.
 3. The method of claim 1 where said fourth step attachmentincludes fiber optic leads from said optical fiber.
 4. The method ofclaim 1 where said fourth step attachment includes a temperature sensor.5. The method of claim 4 where said temperature sensor is an opticalfiber having a Bragg grating.
 6. The method of claim 1 where saidoptical fiber has Bragg gratings, and said first step placing saidoptical fiber includes placing said optical fiber such that said Bragggratings are located between said posts and in areas where stress ofdeflection can be measured.
 7. The method of claim 1 where said opticalfiber has a plurality of Bragg gratings disposed in distinct locationson said fiber, each said Bragg grating operative for a differentwavelength, and said first step includes placing said optical fiber suchthat each said Bragg grating is located between said posts and in areaswhere stress of deflection can be measured in each axis for measurementof deflection.
 8. The method of claim 1 where said posts have a crosssection which is at least one of: hexagonal, square, or triangular.
 9. Amethod for manufacturing a sensor using an outer mold, an inner mold,and an optical fiber having at least one grating formed therein, themethod having the steps: placing said inner mold within said outer moldafter placement of an optical fiber between posts which are either partof said inner mold or part of said outer mold; filling a void betweensaid outer mold and said inner mold with an elastomer; where said atleast one grating is positioned in a region between two adjacent saidposts.
 10. The method of claim 9 where said grating is a fiber Bragggrating.
 11. The method of claim 9 where said optical fiber has at leastthree gratings which are positioned with substantially 120 degrees ofseparation about an axis of said inner mold.
 12. The method of claim 9where said optical fiber has at least four gratings which are positionedwith substantially 90 degrees of separation about an axis of said innermold.
 13. The method of claim 9 where said outer mold and said innermold also form a closed end cap.
 14. The method of claim 9 where saidvoid forms a substantially cylindrical region having an extent.
 15. Themethod of claim 9 where said posts have one of the cross sections ofhexagonal, triangular, or square.
 16. The method of claim 9 where saidplacement of an optical fiber includes placing a wire mesh in said void.17. A method for making a shape and deflection sensor, the method havingthe steps: making an outer form with a substantially cylindrical middlesection; making an inner form for placement inside said outer form andforming a void there between; placing an optical fiber having Bragggratings in the gaps between posts formed in either said outer form orsaid inner form which extend into said void; placing a liquidelastomeric material in the void formed between said outer form and saidinner form; curing said elastomeric material until said elastomericmaterial has hardened; removing said cured elastomeric material fromsaid outer form and said inner form.
 18. The method of claim 17 whereone of said bragg gratings is a temperature measurement grating which isplaced in a non-deflecting region of said elastomeric material.
 19. Themethod of claim 17 where said gratings are fiber Bragg gratings placedin a single optical fiber, each said Bragg grating responsive to aunique range of wavelengths.
 20. The method of claim 17 where said postshave a cross section which is at least one of hexagonal, square, ortriangular.
 21. The method of claim 17 where said placing an opticalfiber step includes a copper mesh in said void.